During non-invasive nasal mechanical ventilation, air or other breathable gas is supplied to a patient through a nose mask. The pressure of the supplied gas is varied during the breathing cycle. A pressure of around 4 to 20 cm H.sub.2 O above atmospheric is generally used during expiration to splint open the upper airway and to improve gas exchange within the lungs. During inspiration, a pressure of around 0 to 40 cm H.sub.2 O above the expiratory pressure is used to assist or replace the patient's own ventilatory efforts. Nasal CPAP, for example, for the treatment of obstructive sleep apnea, generally uses equal inspiratory and expiratory pressures.
A problem experienced by patients undergoing non invasive nasal mechanical ventilation or nasal CPAP treatment is that of air escaping (leaking) via the mouth. This leads to drying and dehydration of the nasal passages, reduction in the pressure of the treatment gas being delivered to the lungs, reduction in the amount of ventilatory assistance, and, where applicable, incorrect triggering ox cycling of ventilatory assistance devices.
Hitherto, there have been several approaches to attempt to solve this problem. One such approach has been the use of a chin strap to hold the jaw of the patient in a closed position. However, air has still been able to leak through the lips and in extreme cases the air cause the lips to flap in a "raspberry" fashion.
A variation of this approach involved adding a thick layer of cloth over the lips of the patient. Any air passing through the patient's lips must be forced through the cloth thereby reducing leakage. The device is uncomfortable and still allows air to leak through the patient's mouth.
Another approach has been to use a resilient mask shaped to the contours of the "average" patient's face. The mask is held in position by a head strap. To deform the mask material to form a fluid tight seal, and to account for variations from the average face, the strap often has to be tightened to an uncomfortable degree which may lead to non-compliance. FIGS. 1 and 2 show a mask 10 of this type being held in place by a strap 12. The mask 10 seals against the face 14 of the patient around the mouth, and the head strap 12 forces the mask 10 against the face 14 in the direction of arrows 16 to press and thereby seal the mask 10 against the face 14.
A variation of the above arrangement involves replacing the resilient mask with a bubble or balloon-like membrane that is inflated by the gas being supplied to the patient. This mask conforms to the contours of the face more readily and thus allows the strap forces to be reduced.
It is an object of the present invention to provide an improved device and/or method for preventing or reducing the passage of air through the mouth, and in particular to provide such a device and method where the force of the head strap may be substantially lessened or even obviated.